The role of the spleen in the febrile response has not been systematically investigated in spite of its fundamental participation in the host defenses against infections. We reported recently that the onset of lipopolysaccharide (LPS)-induced fever, irrespective of its route of administration, is temporally correlated with the appearance of LPS in the liver, and that splenectomy significantly increases both the febrile response to LPS and the uptake of LPS by the liver (Kupffer cells [KC]). To further evaluate the association between the spleen and the liver in LPS fever production, we ligated the connection between them, the splenic vein of guinea pigs and, 7 and 30 days later, challenged the conscious animals with LPS intraperitoneally (ip). Both the febrile response and the uptake of LPS by KC were augmented until new collateral veins developed, suggesting that the spleen may normally contribute a factor that limits the KC uptake of LPS and, thus, modulates the febrile response. To further verify the presence of a splenic inhibitory factor, we prepared an extract of spleens from guinea pigs pretreated ip with LPS or pyrogen-free saline, homogenized and ultrafiltered it, and injected it intravenously (iv) into splenectomized guinea pigs pre-treated with LPS ip. The results confirmed our presumption, viz., the splenic extract from LPS treated guinea pigs inhibited the exaggerated febrile response observed in the Splex guinea pigs, supporting the putative presence of a splenic inhibitory factor. Preliminary data indicates that the factor/s most likely is a lipid. The identity of the splenic factor(s) that may thus be released into the splenic vein, however, is (are) still uncertain, but the fact that the active principle passes through a microporous filter having a nominal molecular weight cutoff of 10,000 d suggests it could be a prostanoid, e.g., PGD2, PGE2, PGF11, thromboxane B2, or a steroid, such as estradiol. These factors occur in the spleen and influence KC activity. The purpose of the present study, therefore, is to substantiate the involvement of the spleen in LPS-induced fever by isolating this factor, and verifying that splenic macrophages are its source. This study will be important for better understanding how the febrile response may be regulated by the spleen to defend health and mitigate the potentially harmful effects of high fevers in infected hosts. PUBLIC HEALTH RELEVANCE The spleen is an important organ of the body. One of its most critical functions is to help prevent infection by bacteria and virus. If the spleen becomes damaged or destroyed (in an automobile accident, for example), the risk of a serious infection becomes much greater, as does the possibility of illness, disease, and even death. Our laboratory has been studying how the spleen protects us from infection by investigating the role of the spleen in the production of fever, one of the first responses to a serious infection. Mild fever can be produced in laboratory animals by treating them with lipopolysaccharide (LPS) a chemical extracted from the cell wall of bacteria. In earlier experiments we found that if we surgically removed the spleen of guinea pigs, LPS caused a much higher fever than it does in animals that still have a spleen. Why does removing the spleen potentiate LPS fever? We think that it involves the liver. Normally, LPS causes fever, at least in part, by stimulating a type of immune cell, the Kupffer cell, which is found in the liver. LPS stimulates Kupffer cells from the inside; that is, LPS must first be taken up into the Kupffer cell before it can cause fever. Our data indicate that the spleen normally protects us against fever by making a chemical that travels through the blood to the liver and tells the Kupffer cells NOT to take up LPS. When we remove the spleen, this splenic factor, as we call it, it not available and Kupffer cells are able to take up a lot more LPS and produce a much higher fever. The objective of our research is to discover the identity of the substance released from the spleen, confirm that Kupffer cells are the target of the splenic factor, and to identify its cell source. To accomplish this, we will treat guinea pigs with LPS to cause the spleen to make the splenic factor, remove the spleen under anesthesia, and then make extracts of the spleen. We will then treat splenectomized guinea pigs with the spleen extracts and see if the extract protects them against the excess of LPS and the exaggerated febrile response of splenectomized animals. With this approach, we should be able to isolate the splenic factor from the extracts that protect guinea pigs against the effects of LPS. In the longer term, if we can identify this splenic factor, we might be able to discover a drug that will reduce fever and protect patients from the dangerous effects of infection. [unreadable] [unreadable] [unreadable]